1. Field of the Invention
The present invention relates to a system capable of rendering more than two density levels at each addressable position on a carrier. The methods of the current invention are especially useful for electrographic printing of continuous tone image by multilevel halftoning.
2. Description of the Prior Art
Most image, text and graphics reproduction systems have the ability to produce continuous density variations or just binary density levels. Photographic black and white prints can produce at every location an almost continuous range of grey levels, ranging from white to black. Photographic colour prints can have at every location a full scala of densities for the different colour components. Offset printers used in the graphic industry on the other hand can render only two density levels of one ink for every location, making it a binary system. At each location, the highest density is printed by full ink coverage or the lowest density is rendered by absence of ink. The illusion of continuous tone must be obtained by screening, as known in the art.
Recently, rendering systems became available that can modulate the density of every location in more than two levels, where this was not possible before. An example for this is an electrographic printer as shown in FIG. 5. In such a system, a latent electrostatic image is first formed on a semiconductor drum 43. The latent image is developed by the application of toner particles 47 that get attracted to or repelled from the drum 43 depending on the local electrostatic load. The toner particles 47 attracted to the drum 43 are then offset on the carrier 37 and fused by pressure and heat onto the carrier 37. This way, the latent image from the drum 43 becomes visible by the density caused by the fused toner particles. For a long time it has been possible in a controlled way to deposit at every location of the semiconductor drum 43 a large amount of toner particles, enough to locally fully cover the carrier 37 or to deposit nothing. In that sense, the electrographic systems were purely binary. This was ideal for rendering text and graphics. The rendering of continuous images could be done by halftoning, as known for offset printing, but with lower spatial and density resolution than offset printing. The local electrostatic load and the deposit of toner particles on the drum can now be controlled more accurately, such that the amount of toner particles at each location on the carrier is more variable. When toner particles on the carrier locally do not cover the carrier 37 completely, grey shades can be obtained. In an electrophotographic printer--a special case of an electrographic printer--the electrostatic load is modulated by a light beam 42. The semiconductor drum 43 is photosensitive and depending on the total light energy impinged on a specific location of the drum 43, the electrostatic load leaks away to ground. By modulation 41 of the light energy, different grey tones or colour tones can be obtained by such a system. Usually, the light energy is applied to individually addressable spots, called microdots 22 as shown in FIG. 3. A system having this capabilities is the Chromapress. This system is marketed by Agfa-Gevaert N. V. from Mortsel Belgium. It is a duplex colour printer (cyan, magenta, yellow, black) having a resolution of 600 microdots per inch producing 1000 A3 pages per hour. Per microdot, 64 different energy levels for the impinged light energy can be selected.
It has been found however that these energy levels do not all produce consistently the same density on the carrier 22 in FIG. 3. For energy levels corresponding with high densities on the carrier, which we will further call high energy levels, there is a predictable relation between energy level and density level on the carrier. For lower energy levels applied to a first microdot, the density level of the first microdot on the carrier strongly depends on the energy level applied to the microdots in the neighbourhood of the first microdot. Therefore, in the example of the Chromapress system, it is not recommended to freely use all 64 available energy levels independent from the energy levels in the neighbourhood. This is referred to as the density level stability.
Moreover, when continuous tone images are presented digitally, mostly 256 different intensity levels are offered. If these 256 intensity levels were mapped directly to the 64 available energy levels, then only 64 density levels would appear on the carrier. Different studies have pointed out that such a poor density resolution results in visually perceptible density steps and contouring. Several attempts have been made--based on classical clustered dot halftoning techniques--to improve the density resolution by screening the intensity levels of the input image signals. But screening reduces the spatial resolution of the system. If for the Chromapress system a screen is used with halftone cells consisting of 6.times.6 microdots, the screen ruling is reduced to 100 lpi (lines per inch) because the basic system has a resolution restricted to 600 dpi (dot per inch). Moreover, most screening techniques known in the art can introduce several types of moire. The first type is auto moire, caused by the interference of the screening pattern with the recorder grid composed of microdots. A second type is subject moire, caused by patterns in the input image that interfere with the screening pattern. A third type is moire caused by overlaying different colour components of a colour image. Each colour component is a halftone image, with a specific screening pattern. When in multi colour reproductions three (cyan, magenta and yellow) or four (including black) single colour halftone images, each having a specific pattern, are superimposed on each other, the specific patterns can interfere with each other and give disturbing moire patterns. In EP 0 240 202 A1 a method is described to distribute energy levels over microdots of small repetitive halftone cells. Due to the equal spacing between energy-concentrated dots, low frequency patterns may appear in the reproduction. There is a need to avoid such disturbing effects.